Low NOx air and fuel/air nozzle assembly

Abstract: An assembly of air and fuel/air nozzles (14,16), the fuel being comminuted coal, are generally of the kind used in a rectangular cross section combustion chamber of an industrial boiler. The air nozzles (14) however, are tilted relative to the fuel/air nozzle (16) so as to diverge therefrom. There results a space between the air flows and the fuel/air flow, which becomes filled with a circulatory flow of the products of combustion. By this means mixing of the air and fuel air flows is delayed and results in a considerable reduction in NOx production.

Description: The present invention relates to a fuel/air nozzle assembly of the kind utilised in the combustion chamber of an industrial boiler. Such nozzle assemblies comprise one or more nozzles which emit a mixture of fuel (coal, gas or oil) and air, and further nozzles which straddle the or each mixed flow nozzle and which emit only air. In some known arrangements all of the nozzles are fixed and in other known arrangements, all of the nozzles are pivotable in a common plane. In both arrangements however, the axes of the nozzles are maintained in parallel with each other. Some of the nozzles described hereinbefore often include such features as flame attachment features, wherein on ignition of the fuel/air mix, the resulting flame attaches to the nozzle outlet plane and extends therefrom into the combustion chamber. Ensuring that this phenomenon occurs achieves a reduction in the production of nitrous oxide. It is also known from EP-A-0399462 to reduce NOx by injecting the oxidant and fuel separately into a combustion zone. The oxidant is injected into the combustion chamber at an angle which diverges away from the axis of the fuel nozzle. To reduce the NOx emissions in the arrangement described in EP-A-0399462 the oxidant is injected with a momentum equal to at least three time the fuel stream momentum. The present invention seeks to provide a simplified air and fuel/air nozzle assembly construction, use of which further reduces NOx production. According to the present invention an air and fuel/air nozzle assembly comprises a first nozzle through which in operation a mixed flow of fuel and air, which defines a fuel/air flowpath, passes to a combustion chamber, and air nozzles which straddle said first nozzle characterised in that each air nozzle is provided with deflectors so angled to deflect air from the air nozzle towards the axis of the air nozzle. The deflectors may be arranged to deflect the air from the air nozzles towards the axis of the air nozzle at an included angle of up to 20° or by an included angle of at least 20°. The assembly may be pivotable so as to enable simultaneous tilting of the nozzles relative to respective air and fuel/air passageway structure to which for operation the nozzles are connected. Preferably the nozzles of the nozzle assembly are interconnected by a linkage to achieve simultaneous tilting. A further aspect of the present invention is a method for reducing NOx production during combustion which comprises conveying a fuel by air such that a mixture of air and fuel is formed and injecting the mixture of air and fuel into a combustion chamber through a first nozzle having an axis, injecting an oxidant into the combustion chamber through an oxidant nozzle having an axis generally parallel to the first nozzle axis, characterised in that the oxidant is deflected at an angle inwardly towards the oxidant nozzle axis such that spaces are formed between the injected mixture of air and fuel and the injected oxidant into which combustion gases are entrained laterally delaying the mixing of the oxidant and the mixture of air and fuel and combusting the mixture of oxidant and fuel. The included angle at which the oxidant is deflected inwardly towards the nozzle angle may be up to 20° or at least 20°. The invention will now be described, by way of example and with reference to the accompanying drawings in which: Fig 1 is a diagrammatic cross sectional side view of a nozzle assembly in accordance with the present invention. Fig 2 is a diagrammatic cross sectional side view of an alternative nozzle assembly in accordance with the present invention. Fig 3 is a graphical representation of the reduction of carbon in a combustion chamber which incorporates the present invention and Fig 4 is a graphical representation of the reduction in NOx in a combustion chamber which incorporates the present invention. Referring to Fig 1. A vertically arranged column of passageways 10 and 12 are defined by a box structure 13 in known manner. Passageways 10 carry respective flows of air to nozzles 14 and a passageway 12 carries a mixture of coal and air to nozzle 16. Passageways 10 and their respective associated nozzles 14 straddle the passageway 12 and its associated nozzle 16, and are tilted relative thereto so that in operation, their flows are caused to diverge from the direction of flow of the coal/air mixture from nozzle 16, in a plane which contains the axes of all of the nozzles 14 and 16. By this action, spaces 18 are formed on each side of the coal/air flow externally of the nozzles, which spaces become filled with combustion gases which are entrained laterally from the furnace. Air and coal/air flows from the nozzles 14 and 16 entrain the combustion gases at the interface therebetween and transport them back to the chamber interior, whereupon adjacent combustion gases in the combustion chamber flow into the spaces 18, thus setting up a flow within the spaces 18. This has the effect of delaying mixing of the airflows from nozzles 14 with the coal/air flow from nozzle 16 and reducing the oxygen content of the mixture arising. The mixing occurs downstream of the outlet plane of the nozzle 16. In addition there is an increase in local turbulence levels and temperature of the gas mixture and the effect of these changes is seen in the graphs shown as figures 3 and 4. In Figure 3 burner testing was effected during which the air nozzles 14 were operated in attitudes ranging from 10° angle of convergence relative to the fuel/air nozzle 16, to 10° angle of divergence. Starting at 10° convergence the percentage of carbon in ash increased until the angle became zero, ie parallel with the nozzle 16. Between 0° and 10° divergence there is a marked drop in the level of carbon in ash due to the improved mixing and increase in mixture temperature. Figure 4 shows how the NOx levels changed for the same variations in air nozzle angle. Over the entire angle change from 10° convergence to 10° divergence the NOx level drops steadily which indicates that the reduction in oxygen is the dominant effect with respect to this parameter. Thus a divergent airflow is able to create conditions leading to lower levels of NOx and reduced levels of carbon in ash simultaneously. The continuing downward trend of the line 22 implies that further NOx reductions are possible if the divergance is taken beyond 10°. Referring now to Fig 2. The air nozzle 24 depicted therein is not intended to be angularly displaced relative to associated fuel/air nozzles 26. Instead it is aligned in parallel therewith, and its outlet end is provided with deflectors 28, which deflect the airflow therefrom towards its axis at an included angle of up to at least 20°. By this means combustion gases 30 are caused to circulate between the air and fuel/air flows. Provided the proportions of the nozzle 24 is sufficient to ensure delivery of the required amount of air, the arrangement depicted in Fig 2 permits the use of one nozzle 24 between fuel/air nozzles 26, rather than two, as would be required in Fig 1. In systems known in the art as tangential firing systems, all of the nozzles are tilted in a common direction. The tilting action is achieved by linkages which may or may not be attached to a unison bar, which in turn, is actuated, eg by ram mechanisms. The nozzles 14, 16, 24 and 26 are also tiltable, about pivot axes 32, 34 and 36, 38 respectively. They are first positioned in the required relative divergent attitudes and then rotated in unison by linkages of the kind described hereinbefore. The fixing would be achieved by using linkages of appropriate relative proportions and connected to the nozzles at appropriate points, all of which is within the capabilities of the ordinary person skilled in the art.

Claims: An air and fuel/air nozzle assembly comprising a first nozzle (26) through which in operation a mixed flow of fuel and air, which defines a fuel/air flowpath, passes to a combustion chamber, and air nozzles (24) which straddle said first nozzle (26) characterised in that each air nozzle (24) is provided with deflectors (28) so angled to deflect air from the air nozzle (24) towards the axis of the air nozzle (24). An air and fuel/air nozzle assembly as claimed in claim 1 characterised in that the deflectors (28) deflect the air from the air nozzles (24) towards the axis of the air nozzle (24) at an included angle of up to 20°. An air and fuel/air nozzle assembly as claimed in claim 1 characterised in that the deflectors (28) deflect the air from the air nozzles (24) towards the axis of the air nozzle (24) at an included angle of at least 20°. An air and fuel/air nozzle assembly as claimed in any of claims 1-3 characterised in that each nozzle (24, 26) of the assembly is pivotable so as to enable simultaneous tilting of the nozzles (24, 26) relative to respective air and fuel/air passageway structures to which for operation the nozzles (24, 26) are connected. An air and fuel/air nozzle assembly as claimed in claim 4 characterised in that the nozzles (24, 26) of the nozzle assembly are interconnected via linkage means for achieving simultaneous tilting. A method for reducing NOx production during combustion comprising, conveying a fuel by air such that a mixture of air and fuel is formed and injecting the mixture of air and fuel into a combustion chamber through a first nozzle (26) having an axis, injecting an oxidant into the combustion chamber through oxidant nozzles (24), each nozzle (24) having an axis generally parallel to the first nozzle axis, characterised in that the oxidant from each nozzle is deflected at an angle inwardly towards its nozzle axis (24) such that spaces are formed between the injected mixture of air and fuel and the injected oxidant into which combustion gases are entrained laterally delaying the mixing of the oxidant and the mixture of air and fuel and combusting the mixture of oxidant and fuel. A method of reducing the production of NOx during combustion as claimed in claim 6 characterised in that the angle is an included angle of up to 20°. A method of reducing the production of NOx during combustion as claimed in claim 6 characterised in that the angle is an included angle of at least 20°.

Inventors: HUFTON PETER FREDERICK, HUFTON, PETER FREDERICK
Applicants: ROLLS ROYCE POWER ENG, ROLLS-ROYCE POWER ENGINEERING PLC
Application year: 1994
Main classification: F23C9
IPC codes: F23C9, F23D1